1. Field of the Invention
The present invention is related to an integrated circuit with transmission lines and a method of manufacturing the same. More particularly, the present invention relates to a highly integrated circuit with transmission lines which have excellent characteristics and a method of manufacturing the same.
2. Description of the Related Art
In an integrated circuit, its speed has been made faster, its frequency has been made higher and its integration density has been made further denser. These factors increase the importance of an integrated circuit with transmission lines between semiconductor devices. Conventionally, such an integrated circuit is known in Japanese Laid Open Patent Application (JP-A-Showa 63-222442).
In this reference, a first grounded metallic element is formed on a GaAs semiconductor substrate. A dielectric substance is formed on the first grounded metallic element. A second metallic element is formed on the dielectric substance. The second metallic element is a signal line. The first and second metallic elements constitute a pair of transmission lines.
The conventional integrated circuit does not achieve a pair of transmission lines having excellent characteristics and highly integrated density at the same time. In the conventional integrated circuit, a film thickness of an interlayer insulating film required to give excellent characteristics to the pair of transmission lines does not belong to a range of a film thickness of an interlayer insulating film allowable for a highly integrated circuit. Its reason will be described below in detail.
A signal attenuation factor D of a pair of transmission lines and a characteristic impedance Z depend on an interval d between a signal line and a ground line of the pair of transmission lines. A signal attenuation factor D and a characteristic impedance Z are respectively represented as follows:                     D        =                  exp          ⁢                      {                          l              ·                              R                int                            ·                              w                                  2                  ⁢                  d                                            ·                                                (                                      ϵ                    μ                                    )                                                  1                  /                  2                                                      }                                              (        1        )                                Z        =                              d            w                    ·                                    (                              μ                ϵ                            )                                      1              /              2                                                          (        2        )            
where 1 is a length of the pair of transmission lines, Rint is a resistance per unit length of a conductor used in the pair of transmission lines, w is a width of the lines, ∈ is a dielectric constant of a dielectric substance between the ground line and the signal line, and xcexc is a magnetic permeability of the dielectric substance.
It is desirable that the signal attenuation factor D of the transmission line is low. In addition, it is desirable that the characteristic impedance Z has a predetermined value. From the consideration of the signal attenuation factor D, the characteristic impedance Z, and the physical constants of materials used in the integrated circuit, it is necessary that the interval d between the signal line and the ground line is around 2 xcexcm . In the conventional integrated circuit, an interlayer insulating film is used as the dielectric substance of the transmission line. Thus, from the viewpoint of the characteristic of the transmission line, it is desirable that the thickness of the interlayer insulating film is about 2 xcexcm.
However, in a highly integrated circuit, it is not allowable that an interlayer insulating film have a thickness of 2 xcexcm. The reason is that a diameter of a contact hole is small in a highly integrated circuit. For example, let us suppose a design rule of 0.25 xcexcm. A diameter of a contact hole based on the design rule of 0.25 m is about 0.4 xcexcm. Here, let us suppose that the thickness of an interlayer insulating film is about 2 xcexcm, which is desirable in an aspect of the characteristics of the transmission line. The aspect ratio of the contact hole is about 5. Such a contact hole is extremely difficult to be formed.
Other integrated circuits having transmission lines are known in Japanese Laid Open Patent Applications (JP-A-Heisei 4-261022, JP-A Heisei 5-22004, JP-A-Heisei 6-85158 and JP-A-Heisei 7-58526). However, any of the integrated circuits does not achieve excellent characteristics of the transmission line with highly integrated structure.
A highly integrated circuit with transmission lines which have excellent characteristics is desirable.
Therefore, an object of the present invention is to provide an integrated circuit which can achieve transmission lines having excellent characteristics and a highly integrated circuit at the same time. Another object of the present invention is to provide an integrated circuit with transmission lines in which the manufacturing process is easy. Still another object of the present invention is to provide an integrated circuit with transmission lines which are not affected easily by the electrical interference.
In order to achieve an aspect of the present invention, an integrated circuit is composed of a substrate, a first conductor formed on the substrate, an insulating film formed on the first conductor and the substrate, a second conductor formed on the insulating film, a first interconnection formed in the insulating film and a second interconnection formed on the insulating film. The first conductor and the second conductor constitute a pair of transmission lines.
The first interconnection and the second interconnection constitute a circuit. The pair of transmission lines and the circuit are separated such that the circuit does not substantially interfere electrically with the pair of transmission lines.
The insulating film includes a plurality of interlayer insulating films. The first interconnection is placed between two of the plurality of interlayer in sulating films.
Thicknesses of the plurality of interlayer insulating films may be defined based on characteristic impedance of the pair of transmission lines.
At least one of the plurality of interlayer insulating films may be formed of dielectric material having a dielectric constant smaller than that of silicon dioxide.
Thickness of the first conductor may be substantially equal to or less than a skin depth determined based on a signal transmitted on the pair of transmission lines.
Thickness of the second conductor may be substantially equal to or less than a skin depth determined based on a signal transmitted on the pair of transmission lines.
The substrate may be made of semiconductor. In this case, a diffusion layer may be formed in surface portion of the substrate. In this case, the first conductor is formed on the diffusion layer.
The substrate may include a semiconductor substrate and a plurality of silicon oxide films which are formed on the semiconductor substrate at the same time. In this case, the first conductor is formed on one of the plurality of silicon oxide films. Remaining ones of plurality of silicon oxide films are used as gate oxide films of MOS transistors.
In order to achieve another aspect of the present invention, a method of manufacturing an integrated circuit is composed of forming a first conductor on a substrate, forming an insulating film on the first conductor and the substrate, forming a first interconnect in the insulating film, forming a first interconnect in the insulating film and forming a second conductor and a second interconnect on the insulating film at the same time. The first conductor and the second conductor constitute a pair of transmission lines, and the first interconnection and the second interconnection constitute a circuit. The pair of transmission lines and the circuit are separated such that the circuit does not substantially interfere electrically with the pair of transmission lines.
The forming an insulating film includes forming a plurality of interlayer insulating films such that the first interconnection is placed between two of the plurality of interlayer insulating films.
Thicknesses of the plurality of interlayer insulating films may be defined based on characteristic impedance of the pair of transmission lines.
At least one of the plurality of interlayer insulating films may be made of dielectric material having a dielectric constant smaller than that of silicon dioxide.
Thickness of the first conductor may be equal to or less than a skin depth determined based on a signal transmitted on the pair of transmission lines.
Thickness of the second conductor may be equal to or less than a skin depth determined based on a signal transmitted on the pair of transmission lines.